Providing power to circuitry in an integrated circuit can be a difficult proposition, especially during power-on (turn on) and power-off (turn off). An integrated header switch is typically used to couple a power supply to the circuitry. To minimize voltage drop across the integrated header switch, the integrated header switch's internal resistance is minimized. However, with a small internal resistance, when the integrated header switch is initially turned on to couple the power supply to the circuitry, a transient current with a large magnitude can result. Damage to the circuitry can result if the magnitude of the transient current is too large. Furthermore, to help ensure chip-level reliability, it is important to ensure that the maximum current draw from the power supply does not exceed normal operating current.
A prior art technique uses multiple switches in an integrated header switch and then turns on each one of the multiple switches sequentially, with a delay between consecutive switches, to help prevent large current spikes. The integrated header switch using multiple switches can be referred to as a distributed header switch. Typically, each of the multiple switches is smaller than the single integrated header switch and therefore, the magnitude of the transient current is smaller when a switch is turned on. Additionally, with a delay between consecutive switches being turned on, the transient current is spread out over time. This can also help to reduce the severity of the transient current.
Another prior art technique makes use of switches with slow gate transitions to prevent large current spikes. The slow gate transitions can effectively reduce the power ramp-up across the switch, thereby reducing the abruptness and the magnitude of the transient current.
One disadvantage of the prior art is that even with smaller transistors being used in the distributed header switches, as each switch is being turned on, a transient current of significant magnitude can still occur. Therefore, with the use of multiple switches, a sequence of transient currents can be produced, each having a magnitude that can be large enough to cause damage.
A second disadvantage of the prior art is that since each of the transient currents produced when a smaller switch is turned on may still exceed the normal operating current, each one of the smaller switches can be replaced with multiple switches that are even smaller. However, due to the delay between turning on consecutive switches, the total amount of time required to provide power to the circuitry can be very long.
Another disadvantage of the prior art is that the use of switches with slow gate transitions is that the design of these switches is essentially an analog design task. This does not scale well to a digital design environment with automatic cell routing and placement, making the design of the integrated header switch more difficult and expensive. Furthermore, changes to the design of the integrated header switch can take longer and may require a substantial re-design of the integrated header switch.